1. Field of the Invention
The present invention relates to an absolute encoder for detecting a rotation angle of a rotary shaft over a plurality of rotations, more particularly to an absolute encoder utilizing reluctance.
2. Description of the Prior Art
It is known that an absolute encoder can detect a rotation angle of a rotary shaft over a plurality of, for example, several hundred rotations. The principle of this detection is to detect the rotation angle of the rotary shaft on the basis of the angles detected by the resolvers attached to the rotary shaft and the other slowed-down rotary shafts for detecting rotation angles during one rotation of the respective rotary shafts.
In conventional absolute encoders, a plurality of resolvers are used and magnetic interference among these resolvers is a problem. That is to say, because a magnetic field generated by a detecting resolver reaches adjacent resolvers, inductance change depending on the rotation angles of other resolvers is added to the inductance change in the detecting resolver itself. Consequently, the detected inductance change actually corresponds to the resultant inductance change. Therefore, satisfactory detecting accuracy can not be obtained. In order to avoid the magnetic interference, it is necessary to arrange the respective resolvers apart at such a distance that the magnetic interference does not occur which results in the problem that the equipment becomes very large.